1. Field of the Invention
The present invention relates generally to the field of quadrupole mass spectroscopy, and more particularly to a method and apparatus for producing and monitoring both positive and negative ions using a quadrupole mass spectrometer.
2. Description of the Prior Art
In quadrupole mass spectrometers, ions of different masses are separated by a quadrupole filter. Although positive and negative ions can be transmitted simultaneously through such a filter, conventionally available devices permit ions of only one polarity to be extracted from the filter for detection and data processing. Normally, only positive ions are detected primarily because commercially available devices are constructed and operated under conditions favoring the generation of positive ions and because electron multipliers are normally operated at negative potentials, thus tending to attract only positive ions and repel negative ions.
Some devices have been constructed which permit sequential detection of positive and negative ions. One such device is sold by Extranuclear Laboratories, Incorporated, of Pittsburgh, Pennsylvania. This device is a quadrupole mass spectrometer which includes a toggle switch for reversing voltage polarities on a single electron multiplier and ion source. A delay of approximately 10 seconds is required between recording ions of different polarities. The delay period required for switching between positive and negative ion detection in these machines is sufficiently long so that simultaneous or near simultaneous recording of ions of both polarities is completely impossible, with the result that accurate mass measurements can only be made in great difficulty and not at all on certain ions with such machines. Similarly, it is not possible to record both positive and negative ion spectra on a single injection of sample molecules introduced into such machines through a gas chromatograph, for example. These factors emphasize the point that sequential detection of positive and negative ions is not at all equivalent to simultaneous, or effectively simultaneous, detection of both polarities of ions. The capability of sequential detection of both types of ions is essentially equivalent to using two separate mass spectrometers to process positive and negative ions, and fails to attain the synergistic effects possible with simultaneous or near simultaneous detection.
It is well understood by those skilled in the art that substantially simultaneous recording of both positive and negative ion species in quadrupole mass spectrometers would be highly desirable in that it would greatly facilitate the making of accurate mass measurements, among other things. In obtaining mass measurements, for example, it is necessary that some means be found to distinguish the ions emanating from an internal standard from those emanating from an unknown sample of nearly the same unit mass as the standard. According to the present invention, distinguishing between ions emanating from the standard and those emanating from the unknown sample is greatly facilitated by operating the mass spectrometer under conditions such that only negative ions, for example, are generated by the internal standard while only positive ions are generated by the sample. Both types of ions are recorded essentially simultaneously using a pulsed ion source, single quadrupole filter, dual electron multiplier detector and dual channel or stereo recording devices with the result that mass measurements with low ppm accuracy can be made with great simplicity.